Capacitor is widely known as an indispensable electronic component for the purpose of power filtering and energy storage in power supply circuits, which convert an AC power such as utility power into a DC voltage tailored to power an electric appliance.
Referring to FIG. 1, a discharge loop in a power supply circuit according to the prior art is shown. As shown in FIG. 1, the conventional power supply circuit 1 includes a filtering capacitor C connected in parallel with the input side of the power supply circuit for removing the high-frequency noises of the input AC voltage Vin, thereby reducing the electromagnetic interference induced during the operation phase of the power supply circuit.
Moreover, the safety regulations of electric appliance normally require the electric energy stored in the filtering capacitor C to be rapidly discharged down to 37% of its peak voltage as the power supply circuit is interrupted in receiving the input AC voltage Vin, thereby reducing the risk of suffering from electric shock as a result of the inadvertent contact of the power supply circuit. In order to meet the safety regulations, the conventional power supply circuit of FIG. 1 includes a discharge loop consisted of a bleeder resistor R connected in parallel to the filtering capacitor C for providing a discharge path for the filtering capacitor C. As the power supply circuit is interrupted in receiving the input AC voltage Vin, the voltage of the filtering capacitor C is able to be discharged through the bleeder resistor R within a time period defined by the time constant which is derived from the multiplication the capacitance of the filtering capacitor C by the resistance of the bleeder resistor R, in order to comply with the standard prescribed by the safety regulations.
With the increasing of the power consumption of electric appliances, the capacitance of the filtering capacitor C and the power consumption of the power supply circuit increase as well. This would deflate the power efficiency of the power supply circuit.
Therefore, the invention proposes an active control circuit for a discharge loop in a power supply circuit to reduce the power loss of the discharge loop and enhance the power efficiency of the power supply circuit.